Rivers, chemical weathering and Earth's climate

We detail the results of recent studies describing and quantifying the large-scale chemical weathering of the main types of continental silicate rocks: granites and basalts. These studies aim at establishing chemical weathering laws for these two lithologies, describing the dependence of chemical weathering on environmental parameters, such as climate and mechanical erosion. As shown within this contribution, such mathematical laws are of primary importance for numerical models calculating the evolution of the partial pressure of atmospheric CO₂ and the Earth climate at geological timescales. The major results can be summarized as follow: (1) weathering of continental basaltic lithologies accounts for about 30% of the total consumption of atmospheric CO₂ through weathering of continental silicate rocks. This is related to their high weatherability (about eight times greater than the granite weatherability); (2) a simple weathering law has been established for basaltic lithologies, giving the consumption of atmospheric CO₂ as a function of regional continental runoff, and mean annual regional temperature; (3) no such simple weathering law can be proposed for granitic lithologies, since the effect of temperature can only be identified for regions displaying high continental runoff; (4) a general law relating mechanical erosion and chemical weathering has been validated on small and large catchments. The consequences of these major advances on the climatic evolution of the Earth are discussed. Particularly, the impacts of the onset of the Deccan trapps and the Himalayan orogeny on the global carbon cycle are reinvestigated. To cite this article: B. Dupré et al., C. R. Geoscience 335 (2003).     

Comparative studies of the kinetic parameters of various algaenans and kerogens via open-system pyrolyses

Kinetic parameters were determined for the first time, via open-system pyrolyses, on algaenans (highly resistant biomacromolecules that are selectively preserved during kerogen formation) isolated from extant microalgae. Parallel studies were also carried out on 10 kerogens exhibiting, with one exception, a low level of maturity. These kerogens included samples chiefly derived from the selective preservation of the above algaenans and samples mainly, or almost exclusively, derived from the “natural vulcanization” pathway. Important differences in activation energy (E_a) distributions were observed between the four algaenans investigated and correlated with their chemical structures. The kerogens predominantly derived from algaenan-selective preservation (Pula alginite, NE 70 and BJ 248 Torbanites, Rundle Oil Shale) also exhibited pronounced differences in E_a distributions. These distributions provided: (i) information on the diversity of the source materials; and (ii) reflected the occurrence of important differences in chemical structures and thermal behaviour between three of the tested kerogens, even though they are all classified as low maturity type I. The Kimmeridge Clay samples and the Lorca Oil Shale showed broad E_a distributions shifted to low energies when compared with the above algaenans and kerogens. Such shifts reflect an important (or even almost exclusive for some of these kerogens) contribution of materials originating from sulphur incorporation into various lipids during early diagenesis. Finally, the kinetic data derived for the nine low maturity fossil samples were extrapolated to a very low, geological heating rate of 3°C Ma⁻¹ and the generation rate curves and cumulative yield curves thus obtained were compared.     

THE RETREAT OF THE FOREST IN SOUTHEAST ASIA: A CARTOGRAPHIC ASSESSMENT

Rapid deforestation is a major problem throughout the tropical world. The conditions and the pace under which societies and economies of the Third World are currently evolving and growing, combined with the specificities of tropical forests, render the latter increasingly vulnerable. Among the major tropical areas of the world, Southeast Asia is perhaps the one where these conditions have had the most impact on the retreat of the forest cover over the last quarter of this century. This is illustrated through the presentation of two maps of the distribution of five basic forest formations in Southeast Asia circa 1970 and circa 1990. The maps are examined and compared, as well as confronted with statistical assessments of deforestation. Finally, the complex causes behind the retreat of the tropical forests as well as the implications of this retreat are briefly discussed.     

The replacement of anthophyllite by jimthompsonite: a model for hydration reactions in biopyriboles

Anthophyllite crystals found in ultramafic lenses of the Lepontine Alps (Switzerland) contain coherent, submicroscopic intergrowths of ordered and disordered biopyribole polysomes. The chain width distributions of disordered polysomes were analyzed using high resolution transmission electron microscopy (HRTEM). Chains wider than triple were interpreted as intermediate products in the transformation of anthophyllite to the triple chain silicate jimthompsonite. The concentration of individual chain types is strongly correlated with the reaction progress. Based on observed zipper terminations and the transformation rules given by Veblen and Buseck (1980) a scheme of possible reaction paths leading from anthophyllite to jimthomp sonite is proposed. The reaction scheme and a simple kinetic model for elementary reactions allow modeling of the observed chain width distributions. The model suggests that the complex reaction paths involving steps with increasing and decreasing chain width are more important in the formation of jimthompsonite than the direct transformation from anthophyllite. The wide chains (>triple) occurring as intermediate products of the multi-step paths are structurally closer to talc than jimthompsonite. The back-transformation of these wide chains to triple chains is, therefore, a strong argument that jimthompsonite is a stable phase and not only a metastable intermediate product in the transformation of anthophyllite to talc. Received: 8 July 1996 / Accepted: 13 December 1996     

地球化学场精细结构解析方案与应用

为满足勘查地球化学对化探数据处理越来越精细的要求，把人工神经网络和统计分析结合起来形成地球化学场精细结构解析方案。该方案包括对地球化学样品的分类和对各类样品性质的详细研究，目的在于提供便捷的数学和计算机软件工具，以提取化探数据中的深层信息。可以用于：①研究各组样品中元素含量水平和组合特征以解释可能的矿种；②研究元素组合特征与指示元素的关联和区别以揭示可能的成矿作用过程；③研究异常样品的空间组合属性以揭示地球化学场的精细结构；④研究异常样品之间的差异性以缩小找矿靶区；⑤综合分析以确定剥蚀深度和找矿前景。该方案也适用于环境地球化学及矿产资源预测研究。     

Bathymetric and geochemical investigation of Kawah Ijen Crater Lake, East Java, Indonesia

A bathymetric survey of Kawah Ijen crater lake was conducted by acoustic sounding in 1996 to compare the lake morphology with those measured in 1922, 1925 and 1938, and to calculate the present lake volume. Even though the lake experienced several hydrothermal eruptions, the maximum depth became shallower (182 m) than before (200 m), resulting in a reduced lake volume (3.0×10⁷ m³).Fifty-two major and minor constituents including rare earth elements and polythionates (PT) of the lake waters at various depths were determined by ICP-AES, ICP-MS and HPLC, respectively. These ions except for several volatile elements are taken up by lake fringe through congruent dissolution of pyroclastics of Kawah Ijen volcano. Most ions are homogeneously distributed throughout the lake, although PT showed a considerable vertical variation. Rare earth elements (REE) in the Kawah Ijen water as well as those from other hyper-acidic crater lakes show distribution patterns likely due to the three rock dissolution (preferential, congruent and residual) types, and their logarithmic concentrations linearly depend upon the pH values of the lake waters.Using the PT degradation kinetics data, production rates of PT, injection rates of SO₂ and H₂S into the lake were estimated to be 114, 86 and 30 tons/day, respectively. Also travel time of the spring water at the Banyupahit Riverhead from Kawah Ijen was estimated to be 600–1000 days through the consideration of decreasing rates of PT. Molten sulfur stocks containing Sn, Cu, Bi sulfides and Pb-barite exposed on the inner crater slope were presumed to be extinct molten sulfur pools at the former lake bottom. This was strongly supported by the barite precipitation temperature estimated through the consideration of the temperature dependence of Pb-chlorocomplex formation.     

Multicomponent ionic diffusion in porewaters: Coulombic effects revisited

The diffusion of an ion in porewaters cannot occur independently of the other ions in solution as a result of Coulombic coupling, as well as from other effects not considered here. Unfortunately, a longstanding disagreement exists about the correct form and meaning of the equations that describe Coulombic coupling in porewaters, i.e., Ben-Yaakov [Am. J. Sci. 281 (1981) 974] vs. Lasaga [Am. J. Sci. 281 (1981) 981]. This paper re-examines this controversy by reformulating the problem starting from fundamental concepts of mass and charge conservation. We show that these antagonistic formulations are both valid and, in fact, equivalent, when the different interpretations of charge balance are resolved. Most of the disagreements between Ben-Yaakov and Lasaga are then shown to result from differing methods of solution, not fundamental disparities in their models. We note, however, that the explanation for the concept of “stationary” gradients of nonreacting ions as given Ben-Yaakov is inaccurate, and such gradients do lead to diffusive fluxes that are counterbalanced by electrochemical migrational fluxes to produce no net flux (excluding advective flux). We further find that the bicarbonate diffusive flux will not balance the diffusional charge flux of sulfate during its reduction if advection is present. This latter imbalance generates compensating fluxes in the other nonreacting ions. We have applied our theory to a simplified case of sulfate reduction in a marine sediment. The results show that nonreacting ions do diffuse and that with normally expected values of porewater advection, the ratio of the bicarbonate to the sulfate flux can be far different than the ideal value of −2.